146 RODENTS OF IOWA 



Genus Castor Linnfeus. 



In addition to the family characters above mentioned, the fol- 

 lowing may be included here, since but a single genus is recog- 

 nized: Upper molars subequal, with one inner and two outer 

 enamel folds; molars with dentinal pulp persisting until late in 

 life; teeth of molar series not parallel but converging anteriorly. 



Dental formula^l. PI ; Pm. 1/1 ; M. 3/3=^20. 



A single form has occurred in Iowa. 



Beaver. 

 Castor carmdensis canadensis, p. 50 



Family MURID/E. 

 RATS AND MICE. 



Lower incisors compressed; premolars wanting; molars never 

 more than three in each jaw, with or without roots, tuberculate 

 or with enamel folds; f rentals contracted; infraorbital foramen 

 wider above than below; .jugal splintlike; postorbital processes 

 wanting; tibia and fibula anchylosed below. 



Key to the Genera of Muridse. 



1. Total length 10.00 inches or over 6 



Total length less than 10.00 inches 2 



2. Front of upper incisors with longitudinal groove 3 



Front of upper incisors without longitudinal groove 4 



3. Superior incisors with a deep longitudinal groove nearly 



as broad as the face of the tooth Reithrodontomys 



Superior incisors longitudinally grooved on outer edges 



only Synaptomys 



4. Tail short; length of tail vertebra" usually not over 2.00 



inches ; form short ; ears short ' . . .Microtns 



Tail longer; length of tail vertebrge over 2.00 inches; form 

 longer ; ears longer 5 



5. Molars of upper jaw with tubercles on crowns extending 



in two longitudinal rows Peromyscus 



Molars of upper jaw with tubercles on crowns extending 



in three longitudinal rows .¥».<; 



6. Tail rounded Rattus 



Tail laterally compressed Fiber 



